Conventionally, there is known a so-called active suspension system which improves the maneuvering capability and the riding comfort of the vehicle by electronically controlling hydraulic or pneumatic actuators arranged between the vehicle body and the wheels. For instance, Japanese patent laid open publication No. 63-11408 discloses an active suspension control system which improves the stability of a vehicle by controlling the actuators so as to change the ratio of loads acting upon the front and rear wheels and thereby modifying its dynamic properties related to turning maneuver depending on the conditions of the intended turning maneuver. Japanese patent laid open publication No. 62-295714 discloses an active suspension system for a vehicle which ensures a stable maneuvering of the vehicle by continually controlling a pressure control valve according to the magnitude of a lateral acceleration or a fore-and-aft acceleration of the vehicle so as to adjust a fluid pressure cylinder and reduces the roll or the yaw motion of the vehicle.
The stability of a vehicle is ensured as long as the cornering forces acting on the front wheel set and the rear wheel set are both large enough to support the vehicle against the lateral inertia force due to the lateral acceleration arising during a turning maneuver. In other words, in order for a four wheeled vehicle to maintain a stable running condition during a turning maneuver, both the front and rear wheel sets must continue to provide sufficiently large cornering forces. The cornering force of each of the wheel sets is given as the sum of the cornering forces of the right and left wheels.
It is well known that the relationship between the maximum cornering force of a vehicle tire undergoing a turning maneuver and its vertical load has a nonlinear property. The maximum cornering force increases in proportion with the increase in the vertical load of the tire when the vertical load is relatively small, but its rate of increase diminishes as the vertical load increases beyond a certain level as can be seen from FIG. 4. In other words, the maximum cornering force of a vehicle tire saturates and cannot increase indefinitely as the vertical load acting on the tire increases beyond a certain limit. Therefore, the total cornering force of either the front wheel set or the rear wheel set bearing a larger part of the overall vehicle weight is critical in determining the overall stability of the vehicle because the wheel set which is closer to the center of gravity of the vehicle is subjected to a larger lateral inertia force due to the lateral acceleration during a turning maneuver and a larger vertical load, and therefore reaches such a saturated part of the cornering force property before the other wheel set. The weight distribution varies depending on the position of the engine and the general layout of the vehicle, and is also affected by the weight of the vehicle occupants and the cargo as well as the acceleration and deceleration conditions of the vehicle.
Further, during a cornering maneuver, the vehicle is subjected to a rolling moment because the lateral inertia force acts upon the vehicle acts at its center of gravity which is located well above the road surface, and this rolling moment shifts the distribution of the load acting on the right and left wheels of each wheel set laterally or between them, and the vertical load acting on the outer wheel becomes greater than that acting on the inner wheel. As can be seen by referring again to FIG. 4, between the total cornering force of each wheel set when the shifting of the load to the outer wheel is small or C.sub.F1i +C.sub.F1o and the total cornering force when the shifting of the load to the outer wheel is large or C.sub.F2i +C.sub.F2o, the relationship C.sub.F1i +C.sub.F1o &gt;C.sub.F2i +C.sub.F2o holds. Therefore, as the difference in the loads acting on the inner and outer wheels increases due to the increase in the lateral inertia force or the centrifugal force and the amount of lateral load shift thereby increases, the overall cornering force of each wheel set diminishes.
Therefore, according to the conventional passive suspension combining a spring and a damper, it was not possible to prevent the drop in the cornering force depending on the state of the turning maneuver of the vehicle, and improvements in the capability of the vehicle for turning maneuver and the stability of the vehicle were difficult to achieve.